About 90 kilometres from the sea, upstream of London, the river begins to exhibit signs of tidal activity as the North Sea begins to affect it. London was reputedly made capital of Roman Britain at the spot where the tides reached in 43 AD, but a variety of factors have pushed this spot up river in the 2000 years since then. The Romans called the river Thamesis. At London, the water is slightly brackish with sea salt.

In the 17th and 18th centuries, during the period now referred to as the Little Ice Age, the Thames often froze over in the winter. This led to the first "Frost Fair" in 1607, complete with a tent city set up on the river itself and offering a number of odd amusements, including ice bowling. After temperatures began to rise again, starting in 1814, the river never again froze over completely.

By the 18th century, the Thames was one of the world's busiest waterways, as London became the centre of the vast, mercantile British Empire. The coming of rail and road transportation, and the decline of the Empire in the years following 1914, have reduced the prominence of the river. London itself is no longer a port of any note, and the Port of London has moved downstream to Tilbury[?]. In return, the Thames has undergone a massive clean-up from the filthy days of the late 19th and early- to mid-20th centuries, and life has returned to its formerly dead waters.

In the early 1980s, a massive flood control device, the Thames Barrier, was opened. It is utilised several times a year to prevent water damage to London's low lying areas upstream.

There are also many tunnels used by "tube" trains as part of the London Underground network or (at Greenwich), the Docklands Light Railway, and a free ferry for vehicles, cycles and people on foot during daylight hours at Woolwich. A further tunnel is under construction as part of a high speed international railway line between Ebbsfleet in Kent and Stratford in Newham[?]. It is due to be completed in 2007 and will be downstream of all older crossings.